1. Field of the Invention
The present invention generally relates to vacuum electron devices and particularly relates to electron current sensing in vacuum electron devices.
2. Background Description
A conventional vacuum electron device comprises an cathode and anode contained in an evacuated envelope. In operation, the cathode is held at a negative potential relative to the anode. Electrons are emitted from the cathode. The potential difference between the cathode and the anode accelerates the emitted electrons from the cathode towards the anode in a beam. A beam current thus flows between the anode and the cathode. In some conventional vacuum electron devices, such as cathode ray display tubes (CRTs), one or more grid electrodes are disposed between the cathode and the anode. In operation, a control voltage is applied to each of the one or more grid electrode(s). The control voltage imposes an electrostatic force on the electron beam. The electron beam current can be adjusted by adjusting the control voltage. The control voltage is typically generated by a control sub-system of the circuit containing the vacuum electron device. In a CRT display, this control sub-system controls the brightness of the picture produced on the screen. In general, such control systems are open loop systems. In such a system, there is no feedback provided between the beam current flowing and the control voltage applied to the grid electrode. In conventional CRT displays, indirect feedback of the beam current is provided by measuring by sensing the current flowing in the anode voltage generator. This technique provides useful control information for controlling a vacuum electron device in which there is only a single electron beam, such as monochrome CRTS. This technique is also suitable for controlling total beam current in a multiple beam vacuum electron device such as a colour cathode ray tube. However, this technique is not suitable for controlling individual beam current in such a device because the individual beam currents are effectively summed at the anode.